1. Field of the Invention
This invention relates to a method for the production of a cubic boron nitride-containing high density inorganic composite sintered article possessing a compact texture and a high hardness. The sintered articles produced by the method of this invention are used as high-rigidity wearproof materials such as, for example, cutting tools and wire drawing dies.
2. Prior Art Statement
Heretofore, cubic boron nitride-containing ceramic sintered articles have been produced by the following method.
Specifically, this production is effected by mixing cubic boron nitride with an inorganic substance and keeping the resultant mixture under an extremely high pressure at an elevated temperature such that the cubic boron nitride will not undergo conversion into a graphite-type phase (hexagonal boron nitride; hBN) an will retain thermodynamic stability sufficient for the sintered article to acquire a compact texture.
The drawing is a phase diagram of boron nitride. In the graph, the area overlying the line 1 represents the thermodynamically stable region for cubic boron nitride and the area underlying the line 1 that for graphite-type boron nitride.
The cubic boron nitride as disclosed in Japanese Patent Public Disclosure SHO 63(1988)-35456, for example, is sintered under a pressure of at least 40 kb at a temperature of at least 1,200.degree. C. These sintering conditions are very harsh and cannot be attained unless a girdle type or belt type ultra-high pressure device is used.
For this reason, the cubic boron nitride containing high-density composite sintered article (hereinafter referred to as "cBN sintered article") is not amenable to easy mass production, entails a high cost of production, and is incapable of being produced in large size.
Wakatsuki et al. conducted an experiment on cubic boron nitride under an ultra-high pressure. They reported that when the cubic boron nitride (hereinafter referred to as "cBN") is in a substantially metastable state though not in a thermodynamically stable state, it remains virtually stably because the time which the cBN requires for being converted into the graphite-type phase state is extremely long and that the highest temperature permitting the retention of this substantially stable state falls o the line 2 shown in the drawing [Wakatsuki, Ichise, Aoki and Maeda: "Program and Abstracts of the 14th High Pressure Conference of Japan", (1972) page 78]. Their observation indicates that even under the conditions of a low pressure and a high temperature falling in the area underlying the thermodynamic equilibrium line 1 of cBN shown in the drawing, the cubic boron nitride exists virtually stably so long as the temperature does not surpass the line 2, for example, is not higher than 1,200.degree. C.
The aforementioned report by Wakatsuki et al. is based on an experiment conducted on cBN only by the use of an ultra-high pressure device. This experiment has demonstrated that the cBN continues to exist without being converted into a graphite-type phase even when it is treated in its virtually metastable state though not in its thermodynamically stable state.
It has been found that the treatment of the kind performed by Wakatsuki et al. does not necessarily require use of the so-called ultra-high pressure device.
In the production of the cBN sintered article with the inorganic compound and the cBN, therefore, application of a pressure effective in promoting the compaction of the inorganic compound enables production of a sintered article having a highly compacted texture.
An object of this invention is to provide a method for the production of a high-rigidity compact cBN sintered article in a substantially metastable, though not thermodynamically stable, region of cBN.
The parent U.S. application Ser. No. 07/436,624 partially meets this object by disclosing and claiming a process related to that disclosed and claimed herein, but wherein pressures are limited to "less than 2,000 MPa, exemplified by conditions under 1,000 MPa. This application address the provision of a process varying pressure above 1,000 MPa and below 2,000 MPa.